Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct, C.sub.2 -C.sub.4 recycle product and a trace C.sub.4.sup.+ nonaromatic byproduct. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction result in rapid catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. This coking tendency makes it necessary to frequently perform catalyst regenerations. In addition, applicants have noted that the prior art catalyst can be deactivated by exposure to hydrogen at temperatures greater than 500.degree. C. Minimizing the deactivation caused by this hydrogen exposure is a particular object of this invention.
Applicants' catalyst contains a zeolite, a gallium component and an aluminum phosphate binder, but is characterized in that it is tolerant to hydrogen exposure at temperatures greater than 500.degree. C. The ability of the catalyst of this invention to withstand extended exposure to hydrogen without significant loss of activity is achieved by treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution. This treatment removes some aluminum and phosphorus (and small amounts of silicon) from the catalyst as evidenced by analysis of the wash water. It is believed that this treatment removes an aluminum/phosphorus species which has deleterious effects on the catalyst when exposed to hydrogen at high temperatures. Since the catalyst is exposed to such conditions during normal operation, the ability to remove such a deleterious species results in the unexpected result of increased catalyst life.